Sheet molding compounds (hereinafter referred to as SMCs) are formed using short reinforcing fibers. Thus, when composite materials are formed by using SMCs, the mechanical properties are lower than those of a prepreg formed by using continuous fibers as reinforcing fibers. Despite such a problem, since SMCs flow when filled into a die during a molding process, SMCs are preferable for making complex shapes with detailed contours, which are hard to follow when using a prepreg formed with continuous fibers.
Among reinforcing fibers, since carbon fibers have the highest specific strength and specific modulus of elasticity, and are capable of forming significantly lightweight molded articles (fiber-reinforced composite material), carbon fibers are used in various applications. While reinforcing fibers in SMCs are being replaced from glass fibers to carbon fibers (Patent Literature 1), technological developments are under way to enhance the appearance of molded articles of carbon-fiber SMCs, and to cap an increase in cost caused by using carbon fibers (achieving lower cost).
One of the measures for achieving lower cost is to prepare SMCs by using fiber bundles composed of 10000 or more single fibers. However, molded articles formed using such SMCs may face problems such as insufficient strength and insufficient elastic modulus even when enough carbon fibers are contained therein. Patent Literature 2 discloses a solution by using carbon fibers having a greater single fiber fineness with preferably a specific roundness.
In addition, it is another option to use both carbon fibers and glass fibers as reinforcing material to be contained in SMCs. However, when glass fibers are also contained, the mechanical properties of the obtained molded articles may be lower than those containing only carbon fibers. To solve such a problem, Patent Literature 3 discloses blended SMCs formed by appropriately adjusting the length of each fiber, the basis weight, and the volume ratios of glass fibers and carbon fibers so as to set their mechanical properties to be at the same level as those of SMCs containing only carbon fibers.